In Situ Spectroscopic Probing of the Hydroxylamine Pathway of Electrocatalytic Nitrate Reduction on Iron-Oxy-Hydroxide

Crystalline γ-FeO(OH) dominantly possessing ─OH terminals (γ-FeO(OH)_c), polycrystalline γ-FeO(OH) containing multiple ─O, ─OH, and Fe terminals (γ-FeO(OH)_pc), and α-Fe₂O₃ majorly containing ─O surface terminals are used as electrocatalysts to study the effect of surface terminals on electrocatalytic nitrate reduction reaction (eNO₃RR) selectivity and stabilization of reaction intermediates. Brunauer-Emmett-Teller analysis and electrochemically determined surface area suggest a high active surface area of 117.79 m² g⁻¹ (ECSA: 0.211 cm²) for γ-FeO(OH)_c maximizing the surface accessibility for nitrate adsorption and exhibiting selective eNO₃RR to NH₃ at pH 7 with a yield rate 18.326 mg h⁻¹ cm⁻², >85% Faradaic efficiency (FE), and at least nine-times catalyst-recyclability. ¹⁵N- and D-labeling combined with in situ IR and Raman studies validate the adsorption of nitrate ions on the ─OH terminals of γ-FeO(OH)_c and the generation of nitrite and hydroxyl amine as eNO₃RR intermediates. A kinetic isotope effect (KIE) value of 2.1 indicates H₂O as the proton source and proton-coupled electron transfer as the rate-limiting step. The rotating-ring disk electrochemical (RRDE) study and subsequent Koutecký-Levich analysis reveal the electron-transfer rate constant (k) for the 2e- reduction of nitrate to nitrite is 5.7 × 10⁻⁶ cm s⁻¹. This study provides direct evidence of the hydroxyl amine formation as the dominant pathway of eNO₃RR on γ-FeO(OH).